1. Field of the Invention
The present invention generally relates to an apparatus for steering a vehicle, and more particularly to a mechanism for providing additional steering or fully automatic steering to steerable wheels of the vehicle.
2. Description of the Prior Art
During vehicle motion, such as cornering, for example, vehicle handling dynamics are affected by various parameters such as road conditions, tire characteristics, weight distribution, braking, accelerating, and the like, which can lead to oversteer or understeer conditions. In an effort to deal with this problem and attempt to maintain vehicle handling dynamics constant, there has been heretofore proposed an adaptive steering control system employing complex suspension link systems, drive train arrangements, and braking systems. In Japanese Patent Laid-open Publication No. 2-106468 which corresponds to the U.S. Pat. No. 4,941,097, there has been proposed an apparatus linked to a steering mechanism for detecting a yaw rate of the vehicle and providing additional control of steering amount in response to the yaw rate.
In the case where an automatic steering control is provided by linking a fluid pressure motor or an electric motor to a conventional steering mechanism, when the automatic steering control is initiated, a steering wheel will be rotated automatically in accordance with a determined steering amount, without manipulation by a vehicle driver. It is preferable that the steering wheel is not rotated unless the vehicle driver manipulates the steering wheel, even in case of the automatic steering control.
In the conventional steering mechanism, a steering shaft, which is rotated in accordance with rotation of the steering wheel, is provided at its tip end with a pinion which meshes with teeth on a rack laterally disposed to be axially movable. Connected to opposite ends of the rack are tie rods which are connected to steering axes of the steerable wheels, respectively. Therefore, if a sub-axial member, similar to the rack for example, is disposed between the rack and each tie rod, and a driving mechanism for adjusting the relative positions of the rack and the sub-axial member is disposed so as to provide the automatic steering control, then it is possible to move the tie rods without rotating the steering wheel to thereby provide the automatic steering control. However, when a reaction force created in the steering control is large, the reaction force can be transmitted to the steering shaft through the sub-axial member and the rack to rotate the steering wheel.